add_reverse_result
Description:
Add a query and corresponding result (or error) to the mock backend, meaning that if it receives a reverse search for params 
      through reverse_resolve (or its asynchronous 
      variants), the mock backend will return the given results or error to the caller.
If a set of params is added to the backend multiple times, the most recently provided results and error 
      will be used.
Exactly one of results and error must be set. Empty result sets are represented as a 
        geocode_error_not_supported error.
Parameters:
| this | |
| results | result set  to return for the query, or null if  | 
| error | error to return for the query, or null if  | 
| params | query parameters to respond to, in the same format as accepted by resolve |